1. Field of the Invention
The invention relates to a process and an apparatus for manufacturing a contact lens.
2. Description of the Related Art
In the manufacture of contact lenses by the so-called full-mould process (casting process), contact lenses are usually produced by dispensing a specific amount of a polymerisable contact lens material into the female mould half. The mould is then closed by putting on the male mould half, a moulding cavity being enclosed between the two moulding surfaces. In that moulding cavity there is the previously dispensed contact lens material, which is polymerised to form an as yet unhydrated contact lens. Subsequently, the mould is opened and the polymerised but as yet unhydrated contact lens is removed and further processed.
Unfortunately, it is not possible as a rule to predict reliably to which of the two mould halves the contact lens will adhere: in some cases it adheres to the male mould half (mould half with the convex moulding surface) and, in others, it stays in the female mould half (mould half with the concave moulding surface). Neither case occurs with sufficient frequency, however, to enable an essentially fully automatic process to be tailored to one of those two cases.
The reasons for this lie inter alia in the manufacture of the mould halves. Such mould halves are usually manufactured by the injection-moulding process. The problems encountered with that process will be briefly explained using the manufacture of the female mould half 1 as an example. In injection-moulding, mould inserts of the kind shown in FIG. 1 are usually used. Such mould inserts F1 belong to the state of the art. The example shown here has the shape of a mushroom. They are produced, for example, by means of the turning technique. When turning the head K of the mushroom, first the surface K1 is turned (FIG. 2), then the surface K2 (FIG. 3). In so doing--because of turning with a turning diamond D--a burr G1 or G2 (FIG. 2, FIG. 3) is produced in each case, which has to be removed when the turning process has been completed. Removal of the burr G1 or G2 is carried out, for example, with a cotton wool bud W (FIG. 4). As a result, at the place where the burr has been removed a rounded area R is produced which deviates from the ideal, burr-free corner, said deviation from the ideal shape being in the range of a few micrometers (FIG. 5). If that mould insert F1 is then inserted and the female mould half 1 is moulded around that mould insert, a projection 10 (FIG. 6), albeit a very small projection, is produced in that area. Depending upon its size, however, when the mould halves are opened that projection may have the result that the lens remains in the female mould half because it catches on the projection, or the lens may slide over the projection so that it is found in the male mould half. It is almost impossible to say which, certainly not with the reliability necessary for an automated process. After opening of the mould, therefore, a check must be made in every case to discover on or in which mould half the polymerised but as yet unhydrated contact lens is located.